1. Field of the Invention
This invention relates to blends of phosphites and phenols and the use of such blends as stabilizers for polymers.
2. Description of the Related Art
A wide range of stabilizers are added to polymers to reduce the detrimental effects of various environments on polymer properties. The stabilizers may be added to the polymers to stabilize them against various sources of degradation such as heat, light, oxygen, mechanical stress and the presence of impurities, any or all of which may be encountered during polymer preparation, storage, shipping, fabrication into useful articles and/or during the ultimate use of those articles. Many polymer degradation pathways are believed to result from the presence of free radicals created in the polymers by, e.g., exposure to heat, ultraviolet radiation, mechanical shear, and/or impurities. It is believed that when a free radical is formed, abstraction of a hydrogen atom from the polymer can initiate a chain reaction that results in polymer oxidation. Subsequent reaction of the radical with an oxygen molecule can yield a peroxy radical, which then can react with an available hydrogen atom to form an unstable hydroperoxide and another free radical. In the absence of polymer stabilizers, these reactions can become self-propagating and can lead to polymer degradation and undesirable reduction in properties.
Antioxidants are polymer additives that are used to reduce oxidative degradation of polymers. Phenolic antioxidants are a commercially significant class of antioxidants in which the phenolic hydroxyl group is flanked by alkyl or arylalkyl groups. The flanking groups provide steric hindrance around the oxygen atom, thereby influencing the performance of the antioxidant. Greater steric hindrance has generally been associated with greater antioxidant activity, and thus in a number of commercially significant phenolic antioxidants the phenolic hydroxyl group is flanked on both sides by relatively large groups. For example, phenolic antioxidants have traditionally been based on structures similar to 2,6-di-t-butyl-4-methylphenol (BHT), in which t-butyl groups symmetrically flank the phenolic hydroxyl group.
Phosphite antioxidants are another commercially significant class of antioxidants. Examples include (bis-(2,4-dicumylphenyl)pentaerythritol diphosphite (DOVERPHOS® S-9228) and tris-(2,4-di-t-butylphenyl)phosphite (IRGANOX® 168). A blend containing 64.1% CYASORB® UV-3529 (1,6-hexanediamine,N,N′-bis(1,2,2,6,6-pentamethyl)-4-piperidinyl)-, polymers with 2,4-dichloro-6-(4-morpholinyl)-1,3,5-triazine), 7.7% CYANOX® 1790 (1,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethyl benzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione), 14.5% IRGAPHOS® 168 (tris-(2,4-di-t-butylphenyl)phosphite), 7.7% ULTRANOX® 626 (bis-(2,4-di-t-butylphenyl)pentaerythritol diphosphite) and 5.1% KADOX® 911 (Zinc Oxide) was apparently sold commercially in 2002. The DOVERPHOS® S-9228, IRGANOX® 168 and ULTRANOX® 626 products are generally considered to be non-symmetrically hindered because the groups flanking the oxygen atom that is attached to the aromatic ring are different from one another.
U.S. Pat. No. 6,770,693 and U.S. Patent Publication No. 2005/0113494 disclose blends of phosphites and hindered phenols as polymer stabilizers. The exemplified blends included a non-symmetrically hindered phosphite (such as DOVERPHOS® S-9228) and a symmetrically hindered phenol (such as tetrakis[methylene (3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane). For blends containing a second phosphite, the optimum ratio of DOVERPHOS S-9228 to the second phosphite was apparently greater than one, indicating that the DOVERPHOS® S-9228 drove the performance of these blends.
There is a need for improved polymer stabilizers, polymer compositions that include such stabilizers, and methods of making such polymer compositions.